1. Field of the Invention
The present invention relates to an idle-mode handoff method in a mobile communication device that is comprised in a cellular radio communication system with base stations, such an idle-mode handoff method being an autonomous handoff method with no involvement of base stations.
2. Description of the Related Art
Cellular radio communication systems are well-known. Such cellular radio communication systems comprise cells or radio zones, together covering a given geographical area. The cells include base stations that, through control and communication channels, establish and maintain communication links with mobile communication devices that are comprised in the cellular radio communication system, and communicate with the mobile communication devices through established communication links.
One type of a cellular radio communication system is a CDMA (Code Division Multiple Access) System as described in the TIA/EIA-98-C Standards document xe2x80x9cTR45 xe2x80x94Recommended Minimum Performance Standards for Dual-Mode Spread Spectrum Mobile Stationsxe2x80x9d, Jun. 16, 1999. In such a system, but also in other systems, radio base stations may communicate with mobile communication devices using slotted mode transmission and reception. Section 3, pp. 3-1 to 3-32 of said Standards document describes CDMA receiver minimum standards, more particularly idle mode handoff processes that are autonomously performed by a mobile station or mobile communication device with no involvement of radio base stations, both for non-slotted and slotted mode transmission and reception. When in the mobile station idle state, in slotted mode, the mobile station searches for a strongest pilot channel signal on a current CDMA frequency assignment during the assigned slots. The mobile station determines that an idle handoff should occur when it detects a pilot channel signal sufficiently stronger than the one it is currently monitoring. The mobile station performs an idle handoff whenever Ec/I0 of a pilot in the neighbor set, i.e., of a neighboring base station, exceeds the Ec/I0 of a pilot in the active set, i.e., the current base station, by 3 dB, Ec/I0 being the pilot strength, Ec being the chip energy, and I0 being interference. When in idle mode, the mobile station receives paging messages from a current base station to which it is synchronized, such paging messages being transmitted in slotted paging channels on a pilot signal. In between reception of paging messages, in order to save power, a mobile station adopts a sleep mode. In such a CDMA mobile station, a so-called searcher determines pilot signal strengths for resolved base station pilot signals, both for the active set and for the neighbor set. Idle handoff uses the determined pilot signal strengths to perform a handoff from the current base station to another base station.
In the PCT patent application WO 99/40745, in a mixed voice and data mobile radio system a call-mode handoff method is disclosed. When a mobile station transmits data to the system, handoff is blocked. Blocking is suspended after termination of the data call, or for after a predetermined time, e.g., after transmission of a whole fax page. The call-mode handoff method requires interaction between the mobile station and the system, i.e., is not autonomous.
It is an object to provide an idle-mode handoff method that avoids interruption of critical jobs in a mobile communication device that need system access.
It is an object of the invention to avoid unqualified blocking of handoffs.
It is another object of the invention to provide an idle-mode handoff method that avoids spurious handoffs due to artifacts in a pilot signal search process, thereby avoiding losing the system and a time consuming reacquisition process.
It is still another object of the invention to provide an idle-mode handoff method that avoids a mobile station to perform idle handoffs in a very rapid succession when being in a location with a number of strong pilots under fading propagation conditions, while performing a critical idle-mode process needing system access.
It is still another object of the invention to provide such an idle handoff that takes into account signal quality measures of pilot signals.
In accordance with the invention, in a mobile communication device comprised in a cellular radio communication system that further comprises a plurality of base stations, an idle-mode handoff method is provided, said method comprising:
receiving paging messages from a first base station; and
initiating an idle-mode handoff process from said first base station to a second base station if it is established that no other idle-mode process with said first base station is being performed.
Other idle-mode processes can be registration or authorization tasks, or the like, requiring system access. The invention ensures that no critical other idle-mode task is interrupted.
Because unqualified blocking of handoffs in a given cell is avoided, the risk of losing the system or of dropped calls is greatly reduced. Particularly in a CDMA system, re-acquisition of the system is a time consuming process that should be avoided, firstly because it is annoying to the user of the mobile communication device, and secondly because it exhausts the battery of the mobile communication device.
Preferably, signal quality measures associated with paging signals, paging message error rates, are also taken into account. Herewith, a higher layer software process, at a software layer higher than the lower software layer that assists the idle handoff process in performing acquisition of the signal quality measures, is informed whether handoff is needed. Preferably, the acquired signal quality measures are provided to the higher layer software process through inter-process communication.
The signal quality measures may be determined in various ways, on the basis of paging message error rates, on the basis of elapsed time in terms of the number of successively received invalid paging messages before receiving a valid paging message, or on the basis of signal strength. The various methods provide a measure for link quality of the paging channels.